The decree was adopted by the cabinet late on Tuesday evening, barely a month since the Social Democrat-led government of Prime Minister Sorin Grindeanu took power.

Romania's top judicial watchdog has filed a constitutional court challenge to the decree, which will take effect in 10 days unless blocked by a court ruling.

Grindeanu showed no sign of giving ground, sending a letter on Wednesday to European Commission head Jean-Claude Juncker explaining why his cabinet chose to pass the decree and a draft bill granting prison pardons for several offences.

The government said it was designed to ease prison overcrowding and bring the criminal code into line with recent constitutional court rulings.

Juncker on Wednesday said he was watching developments with "great concern", warning that the fight against corruption in Romania "needs to be advanced, not undone".

Six western states including Germany and the US have also criticised the government.

"The Social Democrats are testing how far they can go," Laura Stefan, an anti-corruption legal expert said. "If we accept they can approve emergency decrees untransparently, then tomorrow they will adopt others and so on. We will wake up without institutions overnight."

Protests were set to continue on Thursday in the capital Bucharest and dozens of cities across the European Union state, where a crackdown on corruption over the past four years has uncovered myriad conflicts of interest and a pattern of local officials handing out state-funded contracts in exchange for bribes.

More than 250,000 Romanians come out in protest after a controversial law giving pardon to corruption crimes was adopted [AP]